selftests/bpf: verify jset handling in CFG computation

A test case to check if both branches of jset are explored when
computing program CFG.

At 'if r1 & 0x7 ...':
- register 'r2' is computed alive only if jump branch of jset
  instruction is followed;
- register 'r0' is computed alive only if fallthrough branch of jset
  instruction is followed.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250613175331.3238739-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

tools/testing/selftests/bpf/progs/compute_live_registers.c

@@ -240,6 +240,22 @@ __naked void if2(void)
 		::: __clobber_all);
 }
 
+/* Verifier misses that r2 is alive if jset is not handled properly */
+SEC("socket")
+__log_level(2)
+__msg("2: 012....... (45) if r1 & 0x7 goto pc+1")
+__naked void if3_jset_bug(void)
+{
+	asm volatile (
+		"r0 = 1;"
+		"r2 = 2;"
+		"if r1 & 0x7 goto +1;"
+		"exit;"
+		"r0 = r2;"
+		"exit;"
+		::: __clobber_all);
+}
+
 SEC("socket")
 __log_level(2)
 __msg("0: .......... (b7) r1 = 0")